The present invention relates generally to optical lithographic techniques used in the formation of integrated circuits and structures on a wafer. In particular, the present invention relates to a reticle having multiple exposure patterns and its use.
Lithographic methods use multiple images or masks to expose patterns in a resist layer on a semiconductor wafer for the formation of integrated circuits and structures such as processors, ASICS and Dynamic Random Access Memory (DRAM). As manufacturing requirements call for exposure of patterns with smaller and smaller dimensions, it is becoming necessary to employ techniques which permit enhancement of the current performance of the process of photolithography. Multiple successive steps of photolithography, film-growth, deposition and implantation of impurities create a complete integrated circuit with many identical copies on the same wafer. Each copy is known as a die.
As integrated circuits have become smaller in dimensions, the photo lithographic process requires more sophistication in alignment techniques and resolution. Presently, photo lithographic processes utilize an instrument referred to as a stepper which moves and aligns the wafer based on alignment marks on a reticle containing an image or mask such that desired patterns on the wafer are exposed based on the image. The reticle contains one or more images which may be referred to as levels because each image is used to form a level on the wafer. Light of a desired wavelength is either projected through or reflected by a selected image from the reticle to expose the substrate. Phase shifting methods, and electron beams, x-rays and ion beams are also used to pattern wafers.
Initially, each reticle contained only one image for forming one level. Reticles were becoming expensive to make due to the exacting conditions required to form smaller and smaller image lines. At the same time, complex integrated circuits required more and more levels and hence a high number of reticles were required to form them. The cost of the reticles required to form the circuits was becoming great. This trend is continuing as chip densities continue to increase.
One proposed solution to this problem is described in U.S. Pat. No. 4,758,863 entitled Multi-Image Reticle. Multiple images were formed on a reticle that was then rotated to expose the wafer using a different image for each level of an integrated circuit formed on the wafer. While this proposed solution reduced the need to keep changing reticles, it still introduced error into the image alignment process. First, each image had to be properly placed at different angles with respect to each other on the reticle. This introduced a rotational alignment error. Error was also introduced when a reticle was not perfectly centered. Rotation of the mask then produced a radial registration error. In addition, the angle of rotation of the mask holder introduced a further rotational alignment error. These errors made it difficult to properly align each reticle based on alignment images through the use of microscopes and other automatic alignment systems. To overcome rotational errors, a further degree of freedom than just movement in the x and y direction was required.
One further problem with this solution is that it left much space unused on the mask. Up to four images are shown on a reticle. There is wasted space, and a practical limit of about four square images which can be used. If more than four square images are used, they must be located further from the center of the reticle to fit within a slice of the reticle. This would further waste space on the reticle.
There is a need to reduce the alignment error inherent in the use of multiple images on a reticle, and there is a further need to increase the number of images and hence reduce the cost of reticles.
A multi-image reticle comprises a two dimensional array of spaced images or mask patterns arranged in a matrix of controllably spaced rows and columns on a single reticle. The images are aligned consistently in the same direction such that no rotation of the reticle is required to expose the levels on a wafer. The wafer is located on a stepper, which controllably positions the wafer under the desired image of the mask for exposure of a resist on the wafer. In one embodiment, X and Y controlled opaque blades or shutters are used to define a movable aperture to allow exposure to occur only through a selected image on the reticle. In a further embodiment, a focussing device is moved to control which image is used. By controlling which image is used, and accurately positioning the wafer via the stepper, multiple images are accurately and repeatably registered, leading to improvement in dimensions of circuitry and other structures formed on the wafer.
In one embodiment, the images are designed for use with a phase shifting reflective and/or refractive mask. In this embodiment, radiation is reflected by each multi layer image to take advantage of interference patterns which are controlled to obtain desired exposure of the resist. Near the boundaries of exposed resist corresponding to the edge of the images on the reticle, the exposure received may be near zero. This can lead to undesired lines being xe2x80x9cprinted.xe2x80x9d To prevent the printing of such undesired lines, a second image on the reticle, either adjacent to the first, or placed anywhere else on the reticle is positioned to expose a second pattern over the first exposure to remove such lines prior to processing of the resist. Since no additional alignment errors are introduced, excellent registration with the previous exposure is obtained. Further images may also be aligned and exposed prior to processing the resist if desired.
Reticle costs are reduced because nine or more image patterns are located on the same reticle. Misregistration is reduced due to the ability of the stepper to position the target under the correct image using global x and y degrees of freedom without introducing errors through rotation of the reticle or having to load and accurately register a new mask. Optimal use of reticle space is made by use of an array of optimally laid out multi size rectangular images allowing a great reduction in the number of reticles and hence cost and time required to manufacture them. This can be a significant cost savings with respect to the manufacture of complex devices such as memory arrays, ASICs, processors and other integrated circuits.
The invention is particularly useful where multiple exposures are used prior to processing a resist to strip away desired areas. Since the level spacing is tightly controlled, and no unload and load of a new mask is required, registration of the two exposures is extremely consistent. Many different types of lithography may be used, including photolithography using either reflective or refractive photomasks, electron beam lithography, x-ray lithography and ion beam lithography to name a few.